In modern surgery, one of the most important instruments available to medical personnel is the powered surgical tool. Typically, this tool comprises a handpiece in which a motor is housed. Secured to the handpiece is an accessory. The accessory is designed for application to a surgical site on a patient to accomplish a specific medical task. Some powered surgical tools are provided with drills or burs for cutting bores into hard tissue or for selectively removing the hard tissue. Still other powered surgical tools are provided with saw blades as cutting accessories. These tools are used for separating large sections of hard and/or soft tissue. The ability to use powered surgical tools on a patient has lessened the physical strain of physicians and other medical personnel when performing procedures on a patient. Moreover, most surgical procedures can be performed more quickly, and more accurately, with powered surgical tools than with the manual equivalents that preceded them.
The Applicant's Assignee's U.S. Pat. No. 5,888,200, (PCT Pub. No. WO 98/005261,) entitled, MULTI-PURPOSE SURGICAL TOOL SYSTEM, incorporated herein by reference, discloses a surgical tool system designed for a number of different applications. This tool system includes a handpiece in which a motor is housed. A first coupling assembly, also part of the handpiece, for selectively couples the shaft of an accessory to the motor shaft. This handpiece also includes a second coupling assembly. The second coupling assembly selectively secures an attachment to the front end of the handpiece. The second attachment includes an elongated nose as the distalmost portion of the attachment. Often internal to the attachment are bearings. The bearings provide a low friction interface between the moving shaft of the cutting accessory and the attachment nose which is static.
The nose of this type of surgical tool system thus performs two functions. First, some of these shafts tend to be small in size, 5 mm or less in diameter. Owing to the material from which these shafts are made and their size, the shafts are, when exposed to side loading, prone to undesirable bending. This bending occurs as a result of the distal end of the shaft, when pressed against tissue to perform a procedure, is subjected to appreciable side loading. Encapsulating an accessory shaft in a nose prevents this bending.
Secondly, the nose encapsulates the moving shaft. This prevents the shaft from contacting and entraining tissue that is not to be subject to which the cutting accessory is applied. This is especially important when the system is applied to tissue located more than a centimeter from the outer skin of the patient. If the location to which the cutting accessory applied is further below skin level and the rotating shaft is unexposed a practitioner might have to form an incision in the patient that is wider than the diameter of the shaft. This wide incision would be necessary to reduce the likelihood that the moving shaft could inadvertently contact tissue that should not be exposed to the moving shaft.
Present nose assemblies prevent undesirable side bending of accessory shafts and prevent the undesirable exposure of these shafts. However, a disadvantage of some nose assemblies is that, when the accessory is actuated a significant amount of friction induced heat is developed at the locations where the moving shaft abuts the components of the nose. This heat is conducted to exposed surfaces of the nose and to other portions of the accessory, including the exposed tip. When these surfaces of the tool system contact the tissue, the heat is transferred to the tissue. This heat has the potential of damaging healthy tissue that should otherwise not be affected by the procedure. This is especially true with nose assemblies that are relatively small in diameter, less than 1 cm. Owing to the size of these noses, it has proven difficult to fit a bearing assembly able to reduce the quantity of friction induced heat that is generated. Instead, inside this type of nose, the rotating shaft often abuts the inner wall of the nose that defined the bore in which the shaft is disposed.
This undesirable heating is especially prone to occur if the nose is what is referred to as a bent nose. As implied by its name a bent nose is a nose that is formed with a bend. Often this bend is within 3 cm of the distal end of the shaft. A surgeon may want to use a surgical tool with a bent nose for a number of reasons. These reasons all root from the fact that, since the nose is bent, the exposed end of the cutting accessory, the end applied to the tissue, is spaced away from the main body nose. One advantage of the cutting accessory being so spaced from the many body of the nose is that when looking down the nose, it is easier to view this end of the accessory as well as the tissue against which the cutting accessory is applied. Further, there are times when owing to the nature of the procedure being performed the surgeon would rather position the cutting accessory laterally against the tissue than longitudinally. Having the cutting accessory extend sidewise away from the main body of the nose facilitates this type of positioning of the cutting accessory.
A tool system with bent nose, by its very nature is formed with a bore that has a bend in the vicinity of the bend in the nose. The cutting accessory used with this type of tool system has a shaft that must be flexible enough to bend in the bent section of the nose bore. Thus, as a consequence of the nature of the components forming this type of system is that as the shaft rotates in the nose, the section of the shaft in the bent section of the nose would have a tendency to rub against the inner wall of the nose that defines the nose. This movement of the shaft against the static surface can be a significant source of friction induced heating of the nose.
A number of solutions have been proposed to either eliminate or reduce this undesirable heating. As mentioned above, one solution is to fit low friction bearings in the nose to eliminate the friction induced heating. Again, when the nose itself is relatively small in diameter this solution is often commercially impracticable. Another solution is to provide a cutting accessory used with this type of nose with a shaft that is of varying diameter. Specifically, the shaft is designed so the section of the shaft that is seated in the bent section of the bore has a diameter less than the diameters of the sections of the shaft that are seated in the straight section of the bore. When this type of shaft rotates, the narrow diameter section of the shaft has minimal if any contact with the surrounding bore-defining inner wall of the nose. This reduces the frictional heating of the nose in the vicinity of the bend. However, a disadvantage of this design is that the narrow diameter section of the shaft is structurally weaker than the rest of the shaft. When the tool is actuated, owing to this section of the shaft be repeatedly bent, this section of the shaft is subjected to appreciable mechanical stress. The combination of this section of the shaft being structurally weak and being subjected to appreciable bending increases the likelihood that, in the course of a procedure, this section of the shaft will be stressed to the point where the section breaks. Should this event occur, the procedure must be interrupted to both collect the separated parts of the cutting accessory and attached a new cutting accessory to the shaft.
A proposed solution to reduce the undesirable heating of the nose of surgical tool system as well as the accessory disposed in the nose is to flow fluid through or around the nose. This fluid is typically sterile water or saline. A known surgical tool system of this design has a nose into which plural longitudinally extending grooves are formed. A sleeve is disposed over the nose. During a procedure in which this assembly is used, the cooling fluid is flowed through these grooves. The sleeve prevents the fluid from flowing away from the nose until the fluid reaches the distal end of the nose. The fluid functions as a heat sink that extracts the heat from the nose and cutting accessory. The fluid flows the heat away from the tool system. This arrangement reduces the temperatures of the nose and cutting accessory from rising to the level at which when these components contact tissue, the tissue is subjected to potentially damage causing heating.
A disadvantage of the above assembly is that for the assembly to transfer a sufficient quantity of heat away from the nose and cutting accessory, a relatively large volume of irrigating fluid needs to be flowed through the nose. This fluid is discharged from the nose adjacent the site at which the cutting accessory needs to be applied. The presence of this fluid can interfere with the performance of the cutting accessory and obstruct the practitioner's view of the site at which the procedure is being performed. To prevent these undesirable consequences it may be necessary to almost continually apply suction to the site at which the procedure is being performed in order to draw this fluid away from the site. Having to perform this added step contributes to the complexity and/or time required to perform the procedure.
Another solution suggested to compensate for the friction induced heating of the nose of surgical tool system is place a wick of fluid retaining material over the wick. Prior to the use of the tool, a fluid, typically saline or sterile water, is coated over the wick. This fluid serves as a heat sink that absorbs the heat generated by the use of the tool. A disadvantage of this practice is that the fluid can absorb only so much heat before the fluid evaporates and is dispersed into the ambient environment as a gas, typically water vapor. Once the fluid has so evaporated, the wick has little thermal capacity to absorb the friction induced heat. If the surgeon wants to hold continue to maintain the nose below at a temperature below a below a certain level, the procedure needs to be interrupted in order to recoat the wick in a heat absorbing liquid.